The invention relates to a component made of an intermetallic compound consisting of titanium and aluminum or made of alloys of such intermetallic compounds with alloying additions so as to form the base material and with an aluminum diffusion coating on the base material.
This base material has interesting characteristics for the construction of engines. It has mechanical characteristics which are comparable to those of conventional titanium alloys while the specific weight is low, but can be used at significantly higher operating temperatures. However, the ductility of this base material at room temperature is lower and must therefore be improved by the use of alloying elements and heat treatment processes, as they are known from German Patent document DE 30 24 645.
While, in the case of conventional titanium alloys, an oxygen embrittlement in an oxidizing atmosphere begins at temperatures starting at 550.degree. C., in the case of intermetallic compounds made of titanium and aluminum, this temperature is at 700.degree. C. The oxygen embrittlement is disadvantageous because the already low ductility further deteriorates at room temperature and results in a brittleness which is known with respect to ceramic components.
In order to use this base material for components which are subjected to operating temperatures of 700.degree. C., as occur preferably in the case of components in the compressor and turbine range of engines, a closed and no-defect aluminum diffusion coating is required on the high-temperature-stressed component surfaces.
When conventional aluminum diffusion coatings are used on components made of the base material, no closed aluminum diffusion coating is achieved. Disadvantageously, coating defects occur. These coating defects include areas of extremely non-uniform coating thicknesses such as trough-shaped coating structures which have no coating on the bottom of the trough. When the coating is extremely thick, these troughs and defects can be covered with aluminum. However, when the component is stressed, these areas will disadvantageously break open and the aluminum covering will chip off.
It is an object of the present invention to provide a component of the above-mentioned type, and a process for its manufacture, in which no coating defects occur and which can be used at operating temperatures of 700.degree. C.
According to the present invention, this object is achieved in that, between the base material and the aluminum diffusion coating, the component has a closed zone which is close to the surface and has a recrystallization structure.
As determined in comprehensive development work, a closed aluminum diffusion coating grows in an undisturbed and uniform manner only on such a recrystallization structure of an intermetallic compound base material consisting of titanium and aluminum, or of alloys of such intermetallic compounds with or without alloying additions. The advantages of the invention are that the application range of such base materials is significantly expanded, and conventional technologies and processes which are suitable for mass production can be used for producing such components.
In the case of a preferred embodiment of the invention, the intermetallic compound is TiAl. In the case of this base material, it could be determined that crystallites with a high stacking fault density occur in the form of crystallographic twin planes in the crystallite. These crystallites exhibit a plate structure, as has not been observed in the case of conventional titanium alloys. In the case of conventional aluminum diffusion coatings, the twin planes remain uncoated. It is only after a zone is formed which is close to the surface and has a crystallization structure that components made from the base material could be represented with a closed aluminum diffusion coating.
A particularly high density of crystalline plate structures is exhibited by base materials made of alloys from intermetallic compounds with a constituent of TiAl of between 50 and 95% by volume and with a Ti.sub.3 Al constituent of between 5 and 50% by volume. In the case of components made of critical base materials, which have a higher proportion of titanium than TiAl and, therefore, tend to have more oxygen embrittlement, uniformly thick aluminum diffusion coatings can be implemented in an advantageous manner through the use of the closed zone according to the invention. The closed zone is close to the surface and consists of a recrystallization structure.
For improving the ductility of the components made of intermetallic compounds, preferably up to 4% alloying additions made of niobium, tantalum, tungsten, vanadium, or mixtures thereof are contained in the component material.
The depth of the closed zone which is close to the surface and has a recrystallization structure amounts to at least 0.1 .mu.m. A recrystallization structure depth between 1 and 10 .mu.m was found to be practical because it can be prepared in a low-cost manner, preferably by using a cold forming which is close to the surface. Recrystallization structure depths between 0.1 and 1 .mu.m are preferably implemented by laser melting and recrystallizing close to the surface. In the case of recrystallization structure depths of above 100 .mu.m, the risk increases that large-volume crystallites with a plate structure are formed during the recrystallization which would prevent a closed aluminum diffusion coating.
A process according to the present invention for producing the components of the above-mentioned type is achieved by the following process steps. The component is cold-formed or slightly melted in a zone which is close to the surface. The component is then annealed at the recrystallization temperature, and finally an aluminum diffusion coating is applied to the recrystallized zone. This process has the advantage that low-cost process steps are provided which are suitable for mass production so that components can be used in engine construction which are improved in a low-cost manner.
For the surface cold forming, a shot peening or machining of the surface areas of the component to be recrystallized is preferably carried out. During shot peening, the component is blasted by ceramic balls made of Al.sub.2 O.sub.3, by glass beads, or by steel balls. In this case, the crystalline structure of the base material is disturbed and internal stress enters into the surface of the base material. During the subsequent recrystallization annealing below the melting temperature of the material, a finely crystalline recrystallization structure is formed on which an aluminum diffusion layer can grow in an undisturbed manner. For surface areas which are not to be coated, protective measures must be taken during the shot peening such as using covers or screens.
For the machining or cold forming close to the surface, pressure rollers, presses, rollers, striking tools or pressure grinding tools may be used.
Preferably, the recrystallization structure may also be formed by the fact that, in the areas which finally are to be coated with aluminum, the surface of the component is first rastered by using a laser beam. In the process, the surface is slightly melted. This has the advantage that particularly low depths of the recrystallization structure between 0.1 and 1 .mu.m can be implemented and the surface areas can be rastered, melted and recrystallized in a geometrically exact manner without using any additional protective measures.
In the case of a preferred implementation of the process, a recrystallizing and an aluminum diffusion coating is carried out using a heat cycle in that first the component, which is cold-formed on the surface or slightly melted on the surface and solidified, is heated to the recrystallization temperature in a system for aluminum diffusion coating. After the recrystallization has taken place, the temperature is set for the aluminum diffusion coating and the transmitted aluminum-containing gas is supplied at the same time.
This implementation of the process fully utilizes the technical conditions of a system for aluminum diffusion coating because, in such systems, the component can be heated independently of the coating process. In addition, contamination danger is reduced because there is no removal or modification between the recrystallization annealing and the coating. This also reduces the cost of the process.
Preferably, the component is subjected to a reduced pressure or to a protective atmosphere during recrystallization so that the heat cycle to the feeding of the aluminum-containing donor gas takes place under a protective gas or at a reduced pressure. This has the advantage that the component surfaces continue to be protected from impurities and oxidation processes.
The powder pack process is known for the aluminum diffusion coating of structural members made of an iron base alloy, a nickel base alloy or a cobalt base alloy. In addition, many different aluminum donors are used for generating aluminum donor gases. The preferred process for the aluminum diffusion coating is the powder pack process, and an aluminum donor of the ternary alloy Ti/Al/C is used for generating a donor gas. In this case, the carbon constituent has the effect that the residual oxygen concentrations remaining in the powder pack are bound or neutralized by use of carbon monoxide formations or carbon dioxide formations, whereas Ti and Al correspond to the base material and therefore promote the growth process of an aluminum diffusion coating on the base material.
The figures illustrate embodiments for an aluminum diffusion coating of components made of intermetallic compounds of titanium and aluminum.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.